Artificial disc system

ABSTRACT

A total artificial expansile disc and a method for posterior insertion between a pair of vertebral endplates are disclosed. The total artificial expansile disc includes at least one pair of substantially parallel plates that move apart along a first axis, in order to occupy a space defined by the vertebral endplates. In another embodiment, each of substantially parallel plates includes a first plate and a second sliding plate. An expansion device or tool is used to move the substantially parallel pair of plates apart along the first axis. A core is disposed between the pair of plates, and the core permits the vertebral endplates to move relative to one another. A ball limiter or ball extender prevents the core from being extruded from between the substantially parallel plates.

This application is a Continuation of U.S. application Ser. No.12/889,328, filed Sep. 23, 2010, which is a Divisional of U.S.application Ser. No. 11/487,415, filed Jul. 17, 2006, now U.S. Pat. No.7,854,766, which claims priority of provisional application 60/788,720,filed Apr. 4, 2006; the entire contents of all the above identifiedpatent applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a posterior placed total lumbarartificial disc (“PTTLAD”) without supplemental instrumentation, thatuses removable bi-functional screws, sliding expansile plates, andinterchangeable cores which enhance individualized custom-fitting. Inaddition, oblique plate traction spikes are used for enhanced vertebralendplate penetration and incorporation. The present invention alsorelates to artificial total lumbar discs which can be posteriorlyintroduced into the lumbar spinal intervertebral disc space,unilaterally, from either left or right side.

2. Description of the Relevant Art

Cervical and lumbar total artificial discs are entering the clinicalneurosurgical and orthopedic markets. The benefits of these artificialdiscs are well known. They replace diseased discs, and preserve motionsegment mobility. Discogenic and radicular pain are relieved withoutforfeiting segmental mobility, which is typical of traditional anterioror posterior lumbar fusions. Thus it is currently rational to placeprosthetic discs anteriorly where access can be easily obtained, andthey can be secured by a variety of anterior screw fixations. Thistechnology is adequate for single level disc replacement in the cervicalspine. However based on the current anterior cervical prosthetic discscrew fixation methodology its implantation is periodically complicatedby screw failures e.g. partial or complete screw pullouts or breaks, andin most designs it is limited to single level replacement. Furthermore,for lumbar total artificial discs, placement is limited to only the L4/5and L5/S1 disc spaces, and not above, secondary to aortic and vena cavalanatomical restraints. Likewise, for the thoracic spine. Thus far notype of thoracic prosthetic disc device has been reported or described.Furthermore, despite the purported safety of placement of the currentanterior total lumbar artificial discs, there is a significant risk ofretrograde ejaculations in males, and the risk of vascular injury, whichalthough small, is potentially catastrophic if it occurs.

The design of total artificial discs, which began in the 1970's, and inearnest in the 1980's, consists essentially of a core (synthetic nucleuspulposus) surrounded by a container (pseudo-annulus). Cores haveconsisted of rubber (polyolefin), polyurethane (Bryan-Cervical),silicon, stainless steel, metal on metal, ball on trough design(Bristol-Cervical, Prestige-Cervical), Ultra High Molecular WeightPolyethylene (UHMWPE) with either a biconvex design allowingunconstrained kinematic motion (Link SB Charite-Lumbar), or a monoconvexdesign allowing semiconstrained motion (Prodisc-Lumbar). There is also abiologic 3-D fabric artificial disc interwoven with high molecularweight polyethylene fiber, which has only been tested in animals.Cervical and lumbar artificial discs are premised on either mechanicalor viscoelastic design principles. The advantages of mechanical metal onmetal designs including the stainless steel ball on trough design andthe UHMWPE prostheses include their low friction, and excellent wearcharacteristics allowing long term motion preservation. Their majorlimitation is the lack of elasticity and shock absorption capacity. Thefavorable features of the viscoelastic prosthetics include unconstrainedkinematic motion with flexion, extension, lateral bending, axialrotation and translation, as well as its cushioning and shock absorptioncapacity. On the other hand, their long term durability beyond ten yearsis not currently known. Containers have consisted of titanium plates,cobalt chrome or bioactive materials. This history is reviewed and welldocumented in Guyer, R. D., and Ohnmeiss, D. D. “Intervertebral discprostheses”, Spine 28, Number 15S, S15-S23, 2003; and Wai, E. K.,Se'mon, G. P. K. and Fraser, R. D. “Disc replacement arthroplasties: Canthe success of hip and knee replacements be repeated in the spine?”,Seminars in Spine Surgery 15, No 4: 473-482, 2003.

It would be ideal if total lumbar artificial discs could be placedposteriorly allowing access to all levels of the lumbar spine. Also onecould place these devices posteriorly in thoracic disc spaces through atranspedicular approach. Similarly if these devices can be placedanteriorly particularly in the cervical spine without anterior screwfixation, and custom-fit it for each disc in each individual, the easeof placement would reduce morbidity and allow for multilevel discreplacement. Placement of an artificial disc in the lumbar spine ifinserted posteriorly through a unilateral laminotomy by using aclassical open microscopic approach or by using a minimally invasivetubular endoscopic approach would significantly reduce the possibilityof recurrent disc herniation. If placed without facet joint violation,or with only unilateral mesial facetectomy, and the device can purchasethe endplates with spikes there would be no need for supplementalposterior pedicle screw fixation, thus obviating the associatedmorbidity associated with pedicle screws and bone harvesting. To take itone step further, if artificial lumbar discs can be posteriorly placedsuccessfully and safely throughout the entire lumbar spine, everyroutine lumbar discectomy could be augmented by artificial discplacement which would simultaneously eliminate discogenic and radicularpain while preserving flexibility. Furthermore by so doing, theprobability of recurrent herniation plummets, and subsequently the needfor posterior pedicle instrumentation plummets, thereby diminishingoverall spinal morbidity, expenditure, and leading to the overallimprovement in the quality of life.

Presumably up to now, technology is not focusing on posterior placementof total lumbar prosthetic discs because of inadequate access to thedisc space posteriorly. To circumvent this problem others have beenworking on the posterior placement, not of a total prosthetic disc butof a prosthetic disc nucleus (PDN), or essentially a core without acontainer (pseudo annulus). PDNs, which are considered post-discectomyaugmentations, have consisted of one of the following materials: 1)hydrogel core surrounded by a polyethylene jacket (Prosthetic DiscNucleus). Two of these devices have to be put in. There is a very high,38% extrusion rate, 2) Polyvinyl alcohol (Aquarelle), 3) polycarbonateurethane elastomer with a memory coiling spiral (Newcleus), 4) Hydrogelmemory coiling material that hydrates to fill then disc space, 5)Biodisc consisting of in-situ injectable and rapidly curable proteinhydrogel, 6) Prosthetic Intervertebral Nucleus (PIN) consisting of apolyurethane balloon implant with in-situ injectable rapidly curablepolyurethane and 7) thermopolymer nucleus implant. (See the twopublications identified above). The approach of posteriorly placingartificial disc cores appears to be flawed in that: 1) there is a highextrusion rate, 2) it lacks good fixation as does total prostheticdevices that are placed anteriorly, 3) it is restricted only to earlysymptomatically disrupted discs which have only nucleus pulposus but notannulus or endplate pathology, and 4) are contraindicated in discs withan interspace height of less than 5 mm.

The primary advantages of artificial disc placement include thereplacement of diseased discs with prosthetic devices which mimic asmuch as possible healthy natural discs thereby relieving axial andradicular pain without forfeiting segmental mobility. There arecurrently in the orthopedic and neurosurgical markets FDA approvedanteriorly placed artificial total lumbar discs. The major disadvantagesof anterior placement of these devices include vascular injury, bloodloss, and retrograde ejaculation in males.

In our previous copending patent application Ser. No. 11/019,351, filedon Dec. 23, 2004 and Ser. No. 10/964,633, filed on Oct. 15, 2004, whichare herein incorporated by reference, we have described artificialexpansile total discs for placement throughout the entire spine. Therelevant history and prior art of artificial discs are summarized andreviewed there. The artificial discs described in our previous patentapplications expand in two or three dimensions, and have internalexpanding mechanisms which necessitate a bilateral surgical approach forposterior placement into the lumbar spine. In one embodiment of thepresent invention, we have simplified the design by omitting an internalexpansion mechanism, and by having the one-pieced disc plates expand inonly one direction. These modifications make it technically easy toplace with minimal disruption of the normal spinal anatomy and withminimal morbidity. Currently in the spinal market there exist onlyanteriorly placed total artificial lumbar discs. The risks of theanterior placement of these discs are well known and documented, andinclude but are not limited by vascular injury and retrogradeejaculation. Their surgical removal if warranted is technicallychallenging and potentially fatal in extreme circumstances. Our designretains all the benefits of the anterior artificial disc with respect tomotion preservation, and has none of the above mentioned risks. Inaddition we introduce an additional novel safety feature, ball limiters,which prevent extrusion of the ball from the artificial disc, and limitcomplete unrestrained motion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an isometric exploded view of the posteriorly placedtotal lumbar artificial disc (“PPTLA”).

FIG. 2 illustrates an isometric view of the closed unexpanded PPLTAdevice.

FIG. 3 illustrates an isometric view of the PPLTA device with anteriorplate expansion (extension).

FIGS. 4A(1) and 4A(2) illustrate isometric and front views of theinsertable core ball (Embodiment I).

FIGS. 4B(1) and 4B(2) illustrate exploded and cross-sectional views ofan alternative ball/trough system (Embodiment II).

FIGS. 5A, 5B and 5C illustrate the components which act in unison toallow width and height device expansion. They include the bi-functional(height/width) adjustment (BFA) screw (5A), the width adjustment screw(5B), and the intervening slotted worm nut (FIG. 5C).

FIGS. 6A and 6B illustrates the external (FIG. 6A), internal (FIG. 6B),and top (FIG. 6c ) views of the dorsal plate.

FIGS. 7A and 7B illustrate the external (FIG. 7A) and internal (FIG. 7B)views of the ventral plate.

FIG. 8 illustrates an orthographic view of the uni-dimensional expandingartificial disc, embodiment I (UDEAD I).

FIG. 9 illustrates an exploded view of the artificial disc (UDEAD I).

FIGS. 10A and 10B illustrate the external and internal views of theexternal plates of the artificial disc (UDEAD I).

FIGS. 11A and 11B illustrate the side and orthographic views of the ballof the artificial disc UDEAD I).

FIGS. 12A and 12B illustrate the orthographic and exploded views of theball limiters (UDEAD I).

FIG. 13 illustrates the ball with the limiters (UDEAD I).

FIG. 14 illustrates a sample position of the entire artificial disc andhow the ball limiters affect range of motion (UDEAD I).

FIGS. 15A, B, and C illustrate the orthographic, frontal and explodedviews of yet another embodiment of the UDEAD (embodiment II) whichemploys a ball with raised edges instead of ball limiters.

FIGS. 16A and 16B illustrate side and orthographic views of the ballemployed in UDEAD II.

FIGS. 17A and 17B illustrate cross-sections of UDEAD (embodiment II)during lateral bending and flexion/extension.

FIGS. 18A, 18B and 18C illustrate the front, back and exploded views ofthe external insertion device used for UDEAD embodiments I and II.

FIGS. 19A and 19B illustrate a detailed view of the plate insertionsection of the external insertion device and the motion of the wedgedseparator expanding the disc plates.

DESCRIPTION OF PREFERRED EMBODIMENTS

The Medical Device of FIGS. 1-7

Referring now to FIGS. 1-7, the above described problems can be solvedin the lumbar spine by the posterior .insertion of a closed PPLTAD inthe discs space after the performance of a discectomy. After insertionit is expanded in height (the anterior-posterior direction in a standingpatient), and in width (disc space height in a standing patient).

FIG. 1 illustrates an isometric exploded view of the PPLTAD. It consistsof two opposing plates 101, 102 which are preferably titanium or cobaltchromium, each of which is comprised of a dorsal component 101 a, 102 aand ventral component 101 b, 102 b. Sandwiched in between the opposingplates 101, 102 is a removable ball 103 which contacts a trough 104 onthe inner aspects of both opposing plates.

The mechanical crux to the PPLTAD height and width expandability arebased on the interaction of a bi-functional (height/width) adjustment(BFA) screw 105 with a slotted worm nut 106, and a width adjustmentscrew 107 and their unified interactions with the dorsal and ventralaspects of each the opposing plates 101, 102, and with their unifiedinteraction with both opposing plates 101, 102.

Located on the outer aspects of the plates 101, 102 are a series ofobliquely oriented spikes 108. The obliqueness of the spikes 108 hindersextrusion by orientation as well as by traction. We believe that this isa unique design which is not found in other prosthetic disc devices.

FIG. 2 illustrates the PPLTAD in its closed position prior to itsinsertion into the empty disc space.

FIG. 3 illustrates the PPLTAD with an extended (expanded) ventral plate102 b.

FIGS. 4A(1) and 4A(2) illustrate isometric and frontal viewsrespectively of the ball insert 103 (Embodiment I). It consists of anellipsoid core 401 surrounded by a raised edge 402. Upon its insertioninto the PPLTAD when both surfaces of the ball 103 contact the troughs104 of the opposing plates 101, 102 and moves within them, the raisededge 402 prevents ball extrusion with patient movement.

FIGS. 4B(1) and 4B(2) illustrate a different ball/trough embodiment(II). In this embodiment it is the ensconcing trough protrusions 404surrounding the ball 403 and ball overhangs 405 which prevent ballextrusion as opposed to the ball rim (Embodiment I) preventing ballextrusion. This preferably allows for the same degree of lateral flexionand rotation as Embodiment I.

FIGS. 5A, 5B and 5C illustrate close up views of the key components ofthe expansile mechanism. FIG. 5A illustrates a close-up of thebi-functional (height/width) adjustment (BFA) screw 105. It is composedof a screw body 501 with threads 502, a hex slot 503, a neck 504 and acollar 505. This screw 105 is inserted into the open bearings 601 of theinner aspect dorsal plate 102 a (FIGS. 6B and 6C) and the heightadjustment threaded nut 704 and slot 703 of the inner aspect of theventral plate 102 b (FIG. 7B).

The BFA threads 502 of screw 105 are in direct contact with the externalslots 509 of the slotted worm nut 106 (FIG. 5B, and FIG. 1). The slottedworm nut 106 in turn has internal threadings 506 (FIG. 5B) whichaccommodate the external threading 507 of the width adjustment screw 107(FIG. 5C, FIG. 1). The countersunk head 510 of the width adjustmentscrew 107 (FIG. 5C), and the head of the slotted worm nut 106 fit intocorresponding slots 602 on the inner aspect of the opposing dorsalplates 102 a (FIG. 6B).

FIGS. 6A, 6B, 6C, 7A and 7B illustrate a variety of views of the dorsaland ventral plates 102 a, 102 b. They illustrate theirinterrelationship, and their connectivity. The external view of thedorsal plate 102 a (FIG. 6A) illustrates a large mid-line flange, andpositioning flanges 603 on its left and right, which insert into theventral plate slots 701 for the dorsal flanges (FIG. 7A). FIG. 6Billustrates the internal aspect of the dorsal plate 102 a. This has thetrough 104 in a fixed position, and the open bearings 601 for insertionof the BFA screws 105. It also illustrates the slots 602 for either thewidth adjustment screws 107 or the worm nuts 106 in the opposing plates.FIG. 6C is a top view of the dorsal plate 102 a illustrating the openbearings 601 for the BFA screws 105, the spikes 108 and the trough 104.

FIG. 7B illustrates the threaded nuts 704 into which the BFA screws 105are inserted as well as their slots 703 which the bottom aspect of theBFA screws 105 rest upon.

Another possible embodiment of the opposing plates includes making theopposing plates different sizes, and decreasing the sizes of the screws,thus allowing even more lateral flexion.

We will now describe the mechanism of height and width expansion. Theclosed PPLTAD is inserted into the emptied disc space (FIG. 2). Theheight is expanded by turning each of the four BFA screws 105 (FIG. 1).These screws 105 by virtue of being inserted into the height adjustmentthreaded nuts 704 of screws of the ventral plate 102 b (FIG. 7B), andthe open bearings 601 of the dorsal plate 102 a (FIG. 6C), allow gradedsliding of the ventral plate slots 703 vis-a-vis the dorsal plateflanges 603 hence achieving graded separation from each other, i.e.height expansion (FIGS. 1, 3, 6A and 7A). When maximum desired height isachieved, further turning of the BFA screws 105 rotate the worm nut 106which then drives the width adjustment screw 107 against the opposingplate thereby leading to opposing plate separation thus driving theopposing plates 101, 102 into the opposing vertebral endplates via thespikes 108 (FIG. 1). Once the plates 101, 102 are engaged in thevertebral endplates via spike 108 penetration and incorporation, the BFAscrews 105 are turned counter-clockwise thereby disengaging them fromthe inner aspects of the plates 101, 102, and the slotted worm nuts 106.The BFA screws 105, slotted worm nuts 106 and width adjustment screws107 are now removed, having performed their jobs of height and widthexpansion. It is necessary to remove these objects so that the innerball core 401 may interact with the inner troughs 104 and achievecomplete and unhindered flexibility of motion. Different sized ballinserts 401 accommodate for differences in disc space height. Thus oncethe plates 101, 102 of the PPLTAD are inserted and driven into theendplates, the disc height is measured, and the appropriately fittedball 401 is inserted to precisely fit the distance of separation betweenthe opposing troughs 104. This maximizes function, and minimizesextrusion.

The Surgical Method of FIGS. 1-7

The method of posterior insertion of the PPLTAD into the posteriorinterspace can be performed open microscopically, or closed tubularly,using endoscopic and/or fluoroscopic guidance.

After the adequate induction of anesthesia the patient is positioned inthe prone position. A midline incision is made, bilateral lamina areexposed, and bilateral hemi-laminotomies are performed preservingbilateral facet joints so as not to incur instability.

A complete discectomy is performed and the superior and inferiorendplates exposed. The closed PPTLA without the core ball 401 isinserted. The four BFA screws 105 are turned clockwise leading to heightextension of the opposing plates 101, 102 via downward sliding of theventral segments 101 b, 102 b of the plates. The screws 105 are turnedfurther clockwise thereby turning the width adjustment screws 107 viathe turning of the slotted worm nut 106. This drives the opposing plates101, 102 with their outer plate spikes 108 into the ventral endplatessecuring their attachment to the vertebral endplates. Fluoroscopicguidance is used to verify placement of the troughs 104 of the inneraspect of the plates 101, 102 at the center of the endplates so thatthey are at the center of gravity. Once the plates are secured intoposition the BFA screws are turned counterclockwise, thereby disengagingfrom the plates 101, 102 and the worm nuts 106. Once disengaged, the BFAscrews 105 are removed from their slots, and the slotted worm nuts 106and widening screws 107 are disengaged from their inserts. We now havetwo opposing plates 101, 102 with their opposing inner troughs 104engaged in two opposing vertebral endplates. The size between theopposing troughs 104 is measured, and a custom-sized ball 401 is nowinserted in between the troughs 104. The size of the ball 401 is suchthat it will fit substantially perfectly, and hence not dislodge. Thepatient is now closed in routine manner.

This device and method of insertion offer safe posterior lumbarplacement with equal motion preservation compared to anteriorly placedlumbar discs. This PPLTAD can also be adopted for anterior lumbarplacement, and for posterior and anterior placement into thoracic discinterspaces. In our previous patent we have a modified plate shape foranterior cervical disc placement. The mechanism described herein iseasily adapted for cervical artificial discs that do and don't expand inheight. We believe this PPLTAD treats disc disease with significantlydecreased morbidity compared to other current devices, whilst preservingspinal segmental flexibility, and enhancing quality of life.

The Medical Device of FIGS. 8-19

Referring now to FIGS. 8-19, the above described problems can also besolved by inserting a total artificial disc 800 which consists of threeseparate components; two opposing bean shaped plates 801, 802 and aninterposing ball 803 which has ball limiters 804 which prevent ballextrusion (embodiment I), or raised edges which prevent extrusion(embodiment II). FIGS. 8 and 9 illustrate orthographic and explodedviews of the artificial disc 800 (embodiment I). FIGS. 15A-C illustratethe orthographic, frontal and exploded views of Embodiment II. FIGS. 16Aand B illustrate the side and orthogonal views of the ball of embodimentII.

FIG. 10A illustrates the external view of either the superior orinterior plates 801, 802 (embodiments I and II). On the external surfaceof the plate 801 there are three types of spikes 808 to facilitatepenetration and integration into the vertebral endplates. There is oneconical center spike 808 a. Around the peripheral edge of the plate 801are multiple pyramidal spikes 808 c. Surrounding the conical spike areright angled lateral spikes 808 b. Each of the three types of spikes 808is designed to facilitate penetration contoured to the shape of theplate 801 with respect to the vertebral endplate. We are not aware ofany other artificial disc designs which have this feature. Alsoillustrated are the alignment slots 805 which align with an externalinsertion/spreading device 1500 (FIGS. 18-19).

FIG. 10B illustrates the internal view of the superior or inferior plate801. Centrally located is a trough 806 which will interact with the ball803 of this ball/trough designed artificial disc. At the center of thetrough 806 are radial grooves 807 which interact with similar radialgrooves 1100 of the ball 803 (FIG. 11B 15C) facilitating ball/troughcontact.

FIGS. 11A and 11B illustrate the ball 803 design (embodiment I). It hassuperior and inferior domes 1102, 1103. In between the domes 1102, 1103is a groove 1100 for the ball limiters 804. The ball limiter 804 insertsinto the ball groove 1100 (FIGS. 11 and 13). FIG. 12 illustrates thatthe ball limiter 804 is composed of superior and inferior leaflets 1201,1202. At the periphery of these leaflets 1201, 1202 there are raisedbarriers 1203 which limit ball motion and extrusion. After the plates801, 802 are inserted, when the ball 803 and limiters 804 areintroduced, the superior and inferior leaflets 1201, 1202 are alignedwith each other. The inferior leaflet 1201 preferably includes a ballgroove insertion ring 1204. After the ball 803 is inserted the balllimiters 804 are rotated such they are at approx 45-90 degrees angledwith respect to each other (FIGS. 12A and 13). FIG. 14 illustrates asample position of the artificial disc 800. It should be noted that withflexion and translation of the device 800, the raised barriers 1203 ofthe ball limiters 804 are in contact with the superior and inferiorplates 801, 802 thereby limiting unrestrained motion of the ball 803,and prevents ball extrusion.

FIGS. 15A, B and C illustrate orthographic, frontal and exploded viewsof embodiment II. In FIGS. 15A, B and C, a ball 1503 is disposed betweensuperior plate 181 and inferior plate 802.

FIGS. 16A and 16B illustrate the side and orthographic views of the ballof UDEAD (embodiment II). The ball 1503 preferably includes a groove1507 for radiographic material, superior dome 1506, and inferior dome(not shown). The ball 1503 also includes superior raised edge 1504,inferior raised edge 1505 and radial grooves 1508. This ball has raisededges instead of limiters which prevent its extrusion and unrestrainedmotion.

FIGS. 17A and 17B illustrate the motion of the ball insert duringlateral bending, and flexion/extension.

FIGS. 18A, 18B, 18C, 19A and 19B illustrate the insertion device 1800.The superior separator 1801 and inferior separator 1802 (FIGS. 18C and19A and B) have extensions which are shaped exactly like the artificialdisc plates 801, 802 and their cradles 1810, 1812 include alignmentfeatures 1815 that fit into the alignment slots 805 of the plates 801,802 (FIGS. 10A and B and 19A and B). The lateral manipulator 1804 andmedial manipulator 1803 (FIGS. 18A-19A and B) when opened lead tosuperior plate 801 and inferior plate 802 separation, and causesubstantially parallel alignment of superior and inferior plate 801, 802penetration into opposing vertebral bodies. The medial and lateralmanipulators 1803, 1804 are attached by a transmission linkage 1805(FIG. 18C). The action wedge 1806 upon manual opening of the instrument1800 by the surgeon inserting his fingers into the manipulator digitinsert 1807 (FIG. 18A) forces the wedge 1806 down in between thesuperior and inferior separators 1801, 1802 leading to superior discplate 801 and inferior disc plate 802 separation, expansion andpenetration into the superior and inferior vertebral bodies. The medialand lateral manipulators 1803, 1804 include digit inserts 1813, 1814 forthe operator of the tool.

The Surgical Method of FIGS. 8-19

The surgical steps necessary to practice the present invention will nowbe described.

After the adequate induction of anesthesia the patient is positionedprone on a fluoroscopically amenable table. A unilateral hemi-laminotomyis performed. The procedure can be performed microscopically,endoscopically or tubularly in routine manner. A routine discectomy isperformed. The superior and inferior disc plates alignment slots 805 areinserted into the cradles of the insertion device 1800. The nerve rootis gently retracted and the disc plates 801, 802 are inserted into thedisc space attached to the inserting/spreading device 1800. Underfluoroscopic guidance the plates 801, 802 are then placed at the centerof gravity of the vertebral plates i.e. at the anterior-posterior anddorsal-ventral centers. When confirmed radiographically, the surgeonspreads the spreader 1800 which drives the wedge 1806 between theseparators 1801, 1802 (FIG. 10) until the plates 801, 802 havepenetrated and incorporated into the superior and inferior vertebralbodies. The inserter/spreader 1500 is then removed. The opposing plates801, 802 are now substantially perfectly opposed to each other. Thedistance between the superior and inferior troughs 806 are now measured,and the surgeon selects from a selection of balls 803 of differentheights to fit between the plates 801, 802, depending on patient size,etc. Using a forceps or similar instrument the ball 803 with the balllimiters 804 (embodiment I), or the ball with raised edges (embodimentII) are inserted in-between the superior and inferior troughs 806.During insertion of the ball 803 the superior and inferior leaflets1202, 1201 of the limiters 804 are aligned with each other. After theball 803 is inserted, the superior and inferior leaflets 1202, 1201using a forceps are separated to effectively prevent ball extrusion andprevent completely unrestrained motion. After inserting the ball ofembodiment II of FIGS. 15A-17B, the correct sized ball is simplyinserted in between the two plates. The wound is then closed routinely.

The current device can easily be adapted for placement in cervical andthoracic discs. It may also be suitable for multiple level placements.This current device enables the restoration of motion of diseased discswith minimal anatomical destruction and invasiveness, and avoids theserious complications of anteriorly placed discs. Furthermore when ananteriorly placed lumbar disc is removed, it is extremely technicallychallenging. Furthermore the artificial disc is then replaced by afusion device limiting motion. The posterior unilateral placement ofthis device obviates all the above mentioned risks. The device presentedhere is safely implanted avoiding anterior vascular structures andnerves which control ejaculation. It is also easily and safely explantedif necessary. The ease and safety of the insertion of this deviceheralds in a new era of safe and simple artificial lumbar disctechnology.

What is claimed is:
 1. An artificial disc system comprising: an artificial disc comprising: a first plate configured to occupy a space defined by a first vertebral endplate of a spine, the first plate including a first endplate-engaging surface having a first plurality of anchors, a first core-engaging surface positioned opposite the first endplate-engaging surface, and a first perimeter surface extending around a first perimeter of the first plate between the first endplate-engaging surface and the first core-engaging surface, wherein the first core-engaging surface comprises a concave core-engaging portion, wherein the first plurality of anchors comprise a first group of at least four anchors extending from the first endplate-engaging surface on a left side of the first endplate-engaging surface and a second group of at least four anchors extending from the first endplate-engaging surface on a right side of the first endplate-engaging surface, wherein each anchor in the first and second groups of the at least four anchors is positioned on the first endplate-engaging surface adjacent the first perimeter surface of the first plate such that each anchor in the first and second groups of the at least four anchors has an exterior surface that extends continuously from the first perimeter surface; a second plate configured to occupy a space defined by a second vertebral endplate of the spine, the second plate including a second endplate-engaging surface having a second plurality of anchors, a second core-engaging surface positioned opposite the second endplate-engaging surface, and a second perimeter surface extending around a second perimeter of the second plate between the second endplate-engaging surface and the second core-engaging surface, wherein the second plurality of anchors comprise a third group of at least four anchors extending from the second endplate-engaging surface on a left side of the second endplate-engaging surface and a fourth group of at least four anchors extending from the second endplate-engaging surface on a right side of the second endplate-engaging surface, wherein each anchor in the third and fourth groups of the at least four anchors is positioned on the second endplate-engaging surface spaced inward of the second perimeter surface of the second plate such that each anchor in the third and fourth groups of the at least four anchors is entirely offset from the second perimeter surface; and a mobile core sized and configured to be positioned between the first and second plates to space the first plate from the second plate and to permit the first and second plates to move relative to one another, wherein the mobile core has first and second plate-engaging surfaces that engage the first and second core-engaging surfaces of the first and second plates respectively, wherein the first and second plate-engaging surfaces of the mobile core are configured to slide against the first and second core-engaging surfaces of the first and second plates, respectively, wherein the first plate-engaging surface of the mobile core has a convex dome portion shaped to mate with the concave core-engaging portion of the first core-engaging surface of the first plate, wherein the mobile core is engaged with the first and second plates such that the first plate can move with respect to the second plate to accommodate lateral bending, flexion, extension, and rotation, and wherein a plurality of raised barriers or protrusions are positioned with respect to the mobile core to partially limit motion of the mobile core with respect to the first and second plates, wherein the raised barriers or protrusions are positioned inward of the second perimeter surface so as to define a gap between the second perimeter surface and the raised barriers or protrusions on the left and right sides of the second plate.
 2. The artificial disc system of claim 1, wherein the mobile core comprises polyethylene and at least one of the first and second plates comprises cobalt chromium.
 3. The artificial disc system of claim 1, wherein the mobile core comprises polyethylene and at least one of the first and second plates comprises titanium.
 4. The artificial disc system of claim 1, wherein the convex dome portion of the first plate-engaging surface of the mobile core has a first height and a first radius with the first height being less than the first radius.
 5. The artificial disc system of claim 1, wherein the mobile core has a first width along an x-axis from a core front to a core back and a second width along a y-axis from a first core side to a second core side, and wherein the second width is equal to the first width.
 6. The artificial disc system of claim 1, wherein the first and second plates are configured to move with respect to the mobile core such that the first and second plates can have a substantially parallel relative orientation as well as a plurality of nonparallel relative orientations.
 7. The artificial disc system of claim 1, wherein the artificial disc is sized and configured to be a cervical artificial disc to be inserted in a cervical disc space.
 8. The artificial disc system of claim 1, wherein the mobile core comprises one or more projections configured to engage the raised barriers or protrusions.
 9. The artificial disc system of claim 1, wherein: the first plurality of anchors also comprises a fifth group of anchors, in addition to the first group of the at least four anchors and the second group of the at least four anchors, positioned at locations spaced inward of the first perimeter surface, such that each anchor in the fifth group of the at least four anchors is entirely offset from the first perimeter surface; and the second plurality of anchors also comprises a sixth group of anchors, in addition to the third group of the at least four anchors and the fourth group of the at least four anchors, positioned adjacent the second perimeter surface, such that each anchor in the sixth group of the at least four anchors has an exterior surface that extends continuously from the second perimeter surface.
 10. The artificial disc system of claim 1, and further comprising: a surgical tool for inserting the artificial disc between the first and second vertebral endplates, the surgical tool comprising: a handle portion; an elongate insertion portion extending distally away from the handle portion; and an implant holder connected at a distal end of the elongate insertion portion and having first and second portions sized and configured to engage the first and second plates so as to hold the first and second plates relatively firmly during insertion and positioning of the first and second plates between the first and second vertebral endplates in a disc space.
 11. The artificial disc system of claim 10, wherein the first portion of the implant holder is hingedly connected to the second portion of the implant holder such that the first portion of the implant holder can be pivotably actuated with respect to the second portion of the implant holder via the handle portion.
 12. The artificial disc system of claim 10, wherein the surgical tool attaches to the first and second plates without engaging the mobile core.
 13. The artificial disc system of claim 1, wherein each of the first and second plates comprise means for engaging with an implant holder of a surgical tool.
 14. The artificial disc system of claim 1, wherein the exterior surface of each anchor in the first and second groups of the at least four anchors extends continuously from the first perimeter surface such that the exterior surface of the respective anchor and a portion of the first perimeter surface together form an unbent, flat surface.
 15. The artificial disc system of claim 1, wherein each anchor in the first and second groups of the at least four anchors is symmetric about a central dimension that runs through a center of the respective anchor.
 16. The artificial disc system of claim 1, wherein each anchor in the third and fourth groups of the at least four anchors has a saw-tooth shape with triangular-shaped lateral sides and a horizontal top ridge that is parallel to the second endplate-engaging surface.
 17. A method of operating the artificial disc system of claim 10, the method comprising: gripping the first and second plates of the artificial disc via the implant holder of the surgical tool; inserting the artificial disc into a patient along a surgical path via the surgical tool into a cervical disc space between the first and second vertebral endplates of the spine; engaging the first and second vertebral endplates with the first and second plates of the artificial disc, such that the first and second plurality of anchors extend into the first and second vertebral endplates; and releasing the first and second plates of the artificial disc and removing the surgical tool.
 18. The method of claim 17, wherein the first portion of the implant holder is hingedly connected to the second portion of the implant holder such that the first portion of the implant holder can be pivotably actuated with respect to the second portion of the implant holder via the handle.
 19. The method of claim 17, wherein the mobile core has a first width along an x-axis from a core front to a core back and a second width along a y-axis from a first core side to a second core side, and wherein the second width is equal to the first width.
 20. The method of claim 17, wherein the artificial disc comprises a first artificial disc and wherein the cervical disc space comprises a first cervical disc space, the method further comprising performing multiple level placements, including the first artificial disc being inserted into the first cervical disc space and including a second artificial disc that is substantially similar to the first artificial disc being inserted into a second cervical disc space.
 21. The method of claim 17, wherein each of the first and second plates comprise means for engaging with the implant holder of the surgical tool.
 22. A method of operating an artificial disc system, the artificial disc system comprising: an artificial disc comprising: a first plate configured to occupy a space defined by a first vertebral endplate of a spine, the first plate including a first endplate-engaging surface having a first plurality of anchors, a first core-engaging surface positioned opposite the first endplate-engaging surface, and a first perimeter surface extending around a first perimeter of the first plate between the first endplate-engaging surface and the first core-engaging surface, wherein the first core-engaging surface comprises a concave core-engaging portion, wherein the first plurality of anchors comprise a first group of at least four anchors extending from the first endplate-engaging surface on a left side of the first endplate-engaging surface and a second group of at least four anchors extending from the first endplate-engaging surface on a right side of the first endplate-engaging surface, wherein each anchor in the first and second groups of the at least four anchors are positioned on the first endplate-engaging surface adjacent the first perimeter surface of the first plate; a second plate configured to occupy a space defined by a second vertebral endplate of the spine, the second plate including a second endplate-engaging surface having a second plurality of anchors, a second core-engaging surface positioned opposite the second endplate-engaging surface, and a second perimeter surface extending around a second perimeter of the second plate between the second endplate-engaging surface and the second core-engaging surface, wherein the second plurality of anchors comprise a third group of at least four anchors extending from the second endplate-engaging surface on a left side of the second endplate-engaging surface and a fourth group of at least four anchors extending from the second endplate-engaging surface on a right side of the second endplate-engaging surface, wherein each anchor in the third and fourth groups of the at least four anchors are positioned on the second endplate-engaging surface spaced inward of the second perimeter surface of the second plate such that each anchor in the third and fourth groups of the at least four anchors is entirely offset from the second perimeter surface; a mobile core sized and configured to be positioned between the first and second plates to space the first plate from the second plate and to permit the first and second plates to move relative to one another, wherein the mobile core has first and second plate-engaging surfaces that engage the first and second core-engaging surfaces of the first and second plates respectively, wherein the first and second plate-engaging surfaces of the first and mobile core are configured to slide against the first and second core-engaging surfaces of the first and second plates, respectively, wherein the first plate-engaging surface of the mobile core has a convex dome portion shaped to mate with the concave core-engaging portion of the first core-engaging surface of the first plate, wherein the mobile core is engaged with the first and second plates such that the first plate can move with respect to the second plate to accommodate lateral bending, flexion, extension, and rotation, and wherein a plurality of raised barriers or protrusions are positioned with respect to the mobile core to partially limit motion of the mobile core with respect to the first and second plates, wherein the raised barriers or protrusions are positioned inward of the second perimeter surface so as to define a gap between the second perimeter surface and the raised barriers or protrusions on the left and right sides of the second plate; and a surgical tool for inserting the artificial disc between the first and second vertebral endplates, the surgical tool comprising: a handle portion; an elongate insertion portion extending distally away from the handle portion; and an implant holder connected at a distal end of the elongate insertion portion and having first and second portions sized and configured to engage the first and second plates so as to hold the first and second plates relatively firmly during insertion and positioning of the first and second plates between the first and second vertebral endplates in a disc space; the method comprising: gripping the first and second plates of the artificial disc via the implant holder of the surgical tool; inserting the artificial disc into a patient along a surgical path via the surgical tool into a cervical disc space between the first and second vertebral endplates of the spine, wherein the mobile core is placed between the first and second plates after the first and second plates are inserted into the cervical disc space; engaging the first and second vertebral endplates with the first and second plates of the artificial disc, such that the first and second plurality of anchors extend into the first and second vertebral endplates; and releasing the first and second endplates of the artificial disc and removing the surgical tool. 